The present invention relates to an electro-sound transducer and an ultrasonic diagnostic apparatus using it. More precisely the present invention provides an electro-sound transducer protected from acoustic multi-reflection which causes medical error information of an ultrasonic diagnostic apparatus.
The present invention reduces the acoustic multi-reflection by reducing the reflection on the surface of the electro-sound transducer, and involves:
rearrangement of each surface direction of the array of transducer elements for avoiding the reflected sound waves;
an acoustic matching layer attached to a piezo-electric device, thus eliminating the reflected sound waves; and
an acoustic matching surface which divides a face of a piezo-electric device into groups having specified area and acoustic reflection factors to eliminate multi-reflection.
In order to disclose the present invention it is necessary to describe briefly the prior art technology of ultrasonic tomography. The ultrasonic diagnostic apparatus is used mainly for observation with ultrasonic tomograms of the human body. It includes a means to radiate and receive sound waves. The electro-sound transducer is a device to radiate sound waves and to receive sound echoes by converting electric signals to sound power and vice versa, based on the piezo-electric effect using lead zirconate titanate (PZT), for instance.
The technology of focusing and scanning a sound beam has many resemblances to microwave technology. The pulse-echo method is similar to a radar system. When electric pulse signals are applied to the transducer, the transducer radiates a sound pulse toward a target (such as a human body), and receives a sound echo from the target. The received sound echoes are converted to electric signals which have information concerning the distances between the transducer and the targets. The intensity of a reflected sound echo corresponds to the acoustic impedance and the transmission character of a target.
FIG. 1 and FIG. 2 show schematically a prior art probe, which radiates/receives and scans a sound wave using only one transducer element.
In FIG. 1, 101 is a transducer which consists of one transducer element (hereinafter simply referred to as "element 101", etc) and generates a single sound-beam 1001. 101-1 is a transducer mount-base on which three or four elements, for instance, are mounted. The mount 101-1 is rotated to scan within a scanning angular width W1 as indicated by dotted lines. 201 shows a part of a transducer housing called a probe unit. 30 is a target such as a human body. 401 is a window made of acoustically transparent material which has almost same acoustic impedance as target 30 and is equipped on an outer surface of the probe 201. The window 401 is for sealing an acoustically transmissible medium as is mentioned further below, and for contacting to the target 30 to reduce ultrasonic loss between the probe 201 and the target 30. M is a medium made of acoustically transmissible material such as silicon rubber, water, or castor oil which are filled in a space between element 101 and window 401. The medium M has almost the same acoustic impedance as window 401 to reduce ultrasonic loss between element 101 and window 401.
In FIG. 2, 102 is a transducer which consists of one transducer element and generates a single sound-beam 1002, 202 is a probe unit, 402 is a window, and 502 is an acoustic reflector placed in a sound pass between the element 102 and the window 402. The reflector 502 oscillates for scanning the single-beam 1002 within the scan width W2 as indicated by the dotted lines. A sound path between element 102 and window 402 is filled by a medium M as in the foregoing example.
The received electronic signals are usually displayed on a cathode-ray tube synchronizing with the scanning, thus providing visible information (an ultrasonic tomogram) of the sound echo.
Recently array technology has been introduced into the transducer. The array transducer arose owing to the advanced technology of the fabrication and control of a multi-element transducer. The array transducer generates, focuses and scans a synthesized sound beam (SS-beam).
The array transducer is a combination of small transducer elements. The wave-fronts of a single-beam from each element are combined together to form a SS-beam. This SS-beam can be focused or scanned by controlling the phase or sequence of an electric pulse signal applied to each element.
The synthesis of the sound beam of the phase control of the sequential pulse signal applied to each element can be done by an electric delay-line or a sequential switch control circuit. The signals received by each transducer element are processed to produce signals for a display, using the same delay-line or the same sequential switch control circuit mentioned above.
There are two kinds of array transducers, one is a phased array transducer and the other is a linear array transducer.
FIG. 3 shows schematically a typical probe unit having a phased array transducer. In the figure, 203 is a probe unit, 103 is a phased array transducer which is composed of a plurality of transducer elements 301. Each element 1031 is arranged on a plane and installed on the outer wall face of probe 203.
All of element 1031 are activated as the sometime but the phase of an electric pulse signal applied to each element 1031 is controlled to generate and scan the SS-beam 1003 within the scan width W3 indicated by the dotted lines.
A linear array transducer, on the other hand, generates an SS-beam by a sub-group of the elements, such as four or five elements, for instance. This SS-beam is shifted in parallel by shifting the elements of the sub-group one by one along the array of elements of the transducer, by sequentially switching the pulse signals applied to the sub-group elements.
FIG. 4 shows schematically a typical probe unit having a linear array transducer. In the figure, 204 is a probe unit, 104 is a linear array transducer which is arranged on a plane and installed on the outer face of the probe 204, and consists of a plurality of elements 1041.
Sequential switching of pulse signals applied to each element of the sub-group 1042 is controlled by a sequential switch control circuit to generate an SS-beam 1004 and make it shift in parallel as shown by arrow W4 and indicated by dotted lines.
FIGS. 5 and 6 show special probe units of an array transducer using the linear array technique.
FIG. 5 illustrates schematically a probe unit 205 using a concave linear array transducer 105 which has sub-group elements 1052. The sub-groups 1052 substantially generate an SS-beam 1005 which is scanned within a scanning angular width W5 as indicated by the dotted lines. Transducer 105 is located inside the probe 205 in order to scan a target 30 effectively within the scan width W5. Therefore a window 405 and a medium M are required.
This concave linear array system is able to scan a sound beam in a sector like in a phase array system with a high angular resolution. More detail is provided in Japanese Patent Publication No. Jitsukosho 52-41267.
FIG. 6 illustrates schematically a probe unit 206 using a convex linear array transducer 106 which has a sub-group element 1062. Sub-group 1062 generates an SS-beam 1006 and scans within the scan width W6 as indicated by the dotted lines.
An acoustically transmissible medium is filled between a transducer and a window as previously described in FIGS. 1, 2, and 5. This is intended to reduce loss of ultrasonic power, however it is difficult to make the acoustic impedance of the medium and of the window completely equal, so that a part of the radiated sound wave at the surface of the window is reflected back toward the transducer and a part of the sound wave reflected by the surface of the transducer element is reflected again toward the window. Thus acoustic multi-reflection occurs in the acoustic path between the transducer and the window.
Acoustic multi-reflection occurs not only at a window but also at a target. Because, no shown in FIGS. 1 to 6, there are some acoustic boundaries in the human body such as the surface of the skin 31, the boundary 32 of different tissue near the skin 31, etc.
In FIGS. 1 to 6, chain lines 2001, . . . , 2006 show reflected sound waves from the window and boundaries, and in these figures it is evident that multi-reflection will occur at the center part of a scanning angular width in the case of FIGS. 1, 2, 3 and 5, and at the whole scanning angular width in the case of FIGS. 4 and 6.
FIGS. 7(a) to (d) show patterns of received signals. In this figure, the horizontal axis is time T, and the vertical axis is a signal amplitude A.
FIG. 7(a) shows ideal received signals without an influence of multi-reflection. In the figure, 71 is a transmitting pulse, 72 is an echo signal of the window, 73 is an echo signal around the surface of human body (skin 31 and boundary 32), 74 are the echo signals of the inner human body from which medical diagnostic information will be taken.
FIG. 7(b) shows an example of the echo signals from the window 72, and its multi-reflected signals 72-1, 72-2, and 72-3.
FIG. 7(c) shows on example of echo signals from around the surface of the human body 73, and its multi-reflected signals 73-1, 73-2, and 73-3.
FIG. 7(d) shows a combined signal of signals FIG.(a), (b) and (c) which actually appears on display.
From the above explanation, it is evident that a multi-reflection causes misjudgement of diagnostic information from the display. This is the problem for a present ultrasonic diagnostic apparatus.